1. Field of the Invention
The invention relates to an acoustic tool for measuring reflectively within a borehole and, more particularly, to a caliper instrument for use in a measuring while drilling environment utilizing acoustic pulses transmitted within a borehole.
2. History of the Prior Art
It has long been recognized in the oil industry that the collection of downhole data during the drilling operation is of extreme value. Such information improves the efficiency of the drilling operation by providing critical data concerning downhole conditions. For example, it is desirable that a continuous record of borehole size be provided so that variations in borehole diameter as a function of depth may be recorded for analysis in connection with the operation of oil wells and the like.
Acoustic well logging is also used in the geophysical and seismic arts to provide surveys of the various formations traversed by the borehole. In particular, acoustic velocity measurements provide valuable information concerning the type of rocks and the porosity thereof in the formation surrounding the borehole. The most commonly measured acoustic parameter in the field of well logging has been the velocity of compression waves. The velocity of shear waves and acoustic impedance have also been of value in determination of both the formation characteristics and the fluid environment.
A myriad of acoustic logging systems for downhole measurements are available in the prior art. One of the most critical measurement parameters of such acoustic logging systems is the acoustic velocity in the the fluid through which the acoustic pulse is transmitted. A high degree of resolution in the interpretation of pulse data is only possible with a precise knowledge of the acoustic velocity in the medium of measurement. Moreover, a high degree of resolution is necessary for the accurate identification of various formation strata as well as other critical borehole parameters.
Many prior art attempts to provide accurate acoustic logging instrumentation have encountered serious problems due to the downhole environment. For example, the drilling operation necessitates the flow of high pressure drilling mud which is pumped down through a central bore in the drill pipe, out through apertures in the drill bit and back to the surface through the annular space between the drill pipe and the side walls of the borehole. The mud removes drill bit cuttings and the like and can reveal much information about the formation itself. Such a fluid system, by definition, includes wide variations in drilling mud density and character both along the borehole as well as in a direction across the borehole annulus. For example, gas present in the drilling fluid has a direct bearing on acoustic velocity within the fluid and the presence of gas varies with position and pressure within the borehole.
One prior art technique of determining acoustic velocity includes sampling the drilling mud at the wellhead for purposes of measurement. However, such a measurement cannot accurately reflect the varying conditions of the mud downhole where the acoustic measurements are actually made. Downhole acoustic pulse data is generally generated by acoustic transducers disposed within the side walls of a sub secured above an operating drill bit within the borehole. The acoustic pulses are transmitted from the sub to the sidewalls of the borehole through the drilling fluid and the reflection time thereof is monitored. The presence of gas or cuttings within the fluid as well as downhole pressures and turbulence thus has a direct bearing on the acoustic velocity and the reflectivity measurements. However, the most convenient location for measuring acoustic velocity is at the wellhead in the passive fluid collection area where the dynamic turbulent downhole conditions are not present. In addition, once received from the borehole, the drilling mud is generally allowed to settle and/or is passed through an out-gassing unit prior to its collection and recirculation. This step drastically alters the acoustic velocity parameters of the drilling fluid from its downhole gaseous and turbulent condition and leads to inaccuracies in the interpretation of the downhole acoustic reflectivity measurements.
A prior art method of overcoming the problems of accurate data collection in a measuring-while-drilling environment is the recording of acoustic borehole measurements with a wireline logging tool. Such tools are utilized with the drill string removed from the borehole and the drilling mud being in a settled state. Such a condition lends itself to a more homogeneous configuration and the presence of mud cakes and turbulence related to nonhomogeneous regions are generally eliminated. One such acoustic caliper logging device is set forth and shown in U.S. Pat. No. 3,835,953 to Summers wherein a wire line tool is provided for positioning within a borehole. A transducer unit repeatedly generates an acoustic pulse as the transducer system is rotated to scan the walls of the borehole in a full circle. A scan of between 1 and 10 revolutions per second may be provided with the tool itself being generally centered within the borehole. The reflections of acoustic energy from the borehole wall are then from a small, centralized area whereby the system can be highly definitive of the character of the wall. Such information is obviously useful in an analysis of the borehole configuration. One distinct disadvantage is, however, the necessity of pulling the drill string from the borehole for utilization of the wireline tool. This operation is both time consuming and expensive from the standpoint of the drilling operation.
In addition, prior art wireline acoustic parameter measurement techniques have obtained acoustic velocity at a downhole location but the acoustic path over which the velocity measurements are made is different from the path over which the parameter is measured. For example, an acoustic caliper measurement made across a borehole annulus which relies on acoustic velocity data obtained in a direction parallel to the borehole axis will not be precise because of the nonlinearity of the flow pattern and flow densities across the borehole.
It would be an advantage, therefore, to overcome the problems of the prior art by providing detailed acoustic caliper information of a borehole in a measuring while drilling configuration. This gives the driller immediate feedback as to the quality of the borehole being drilled and can be used to infer insitu stresses. The method and apparatus of the present invention provide such a system by utilizing a series of acoustic transceivers disposed both laterally and longitudinally upon a drill string sub for use in a measurement while drilling mode. The acoustic transceivers further measure the drilling fluid acoustic velocity simultaneously with the measurement of distance, in the same location in the borehole as the desired distance measurement and along a portion of the same acoustic path as the distance measurement. This provides acoustic reflectivity data based upon an acoustic velocity measurement and a distance measurement generated with a common acoustic pulse. In this manner the accuracy of the data is much higher and more reliable than conventional prior art techniques.